Not applicable.
The present invention relates to a laser-based system or method for severing integrated circuit (IC) device fuses, and, in particular, to such a system or method that employs a single UV laser pulse to sever an IC fuse.
FIGS. 1, 2A, and 2B show repetitive electronic circuits 10 of an IC device on a wafer or workpiece 12 that are commonly fabricated in rows or columns to include multiple iterations of redundant circuit elements 14, such as spare rows 16 and columns 18 of memory cells 20. With reference to FIGS. 1, 2A, and 2B, circuits 10 are also designed to include particular laser severable circuit fuses or links 22 between electrical contacts 24 that can be removed to disconnect a defective memory cell 20, for example, and substitute a replacement redundant cell 26 in a memory device such as a DRAM, an SRAM, or an embedded memory. Similar techniques are also used to sever links to program logic products, gate arrays, or ASICs.
Links 22 are about 0.5-2 microns (xcexcm) thick and are designed with conventional link widths 28 of about 0.8-2.5 xcexcm, link lengths 30, and element-to-element pitches (center-to-center spacings) 32 of about 2-8 xcexcm from adjacent circuit structures or elements 34, such as link structures 36. Although the most prevalent link materials have been polysilicon and like compositions, memory manufacturers have more recently adopted a variety of more conductive metallic link materials that may include, but are not limited to, aluminum, copper, gold nickel, titanium, tungsten, platinum, as well as other metals, metal alloys, metal nitrides such as titanium or tantalum nitride, metal silicides such as tungsten silicide, or other metal-like materials.
Traditional 1.047 xcexcm or 1.064 xcexcm infrared (IR) laser wavelengths have been employed for more than 20 years to explosively remove circuit links 22. Before link processing is initiated, circuits 10, circuit elements 14, or cells 20 are tested for defects, the locations of which may be mapped into a database or program that determines locations of links 22 to be processed. Typically, the same IR laser beam used for processing the links is used, at reduced intensity, to locate the position of the focused spot of the IR laser beam with respect to reflective alignment marks, such as metal on oxide, positioned at the corners of the dies and/or wafers supporting the electronic components.
Conventional memory link processing systems focus a single pulse of IR laser output having a pulse width of about 4 to 20 nanoseconds (ns) at each link 22. FIGS. 2A and 2B show a laser spot 38 of spot size diameter 40 impinging a link structure 36 composed of a polysilicon or metal link 22 positioned above a silicon substrate 42 and between component layers of a passivation layer stack including an overlying passivation layer 44 (shown in FIG. 2A but not in FIG. 2B), which is typically 2000-10,000 angstrom (A) thick, and an underlying passivation layer 46. Silicon substrate 42 absorbs a relatively small proportional quantity of IR radiation, and conventional passivation layers 44 and 46 such as silicon dioxide or silicon nitride are relatively transparent to IR radiation. FIG. 2C is a fragmentary cross-sectional side view of the link structure of FIG. 2B after the link 22 is removed by the prior art laser pulse. The quality of the crater formed in FIG. 2C is neither uniform nor predictable.
To avoid damage to the substrate 42 while maintaining sufficient energy to process a metal or nonmetal link 22, Sun et al. in U.S. Pat. No. 5,265,114 and U.S. Pat. No. 5,473,624 proposed using a single 9 to 25 ns pulse at a longer laser wavelength, such as 1.3 xcexcm. to process memory links 22 on silicon wafers. At the 1.3 xcexcm laser wavelength, the absorption contrast between the link material and silicon substrate 42 is much larger than that at the traditional 1 xcexcm laser wavelengths. The much wider laser processing window and better processing quality afforded by this technique has been used in the industry for several years with great success.
The 1.0 xcexcm and 1.3 xcexcm laser wavelengths have disadvantages however. In general, the optical absorption of such IR laser beams 12 into a highly electrically conductive metallic link 22 is less than that of visible or UV beams; and the practical achievable spot size 38 of an IR laser beam for link severing is relatively large and limits the critical dimensions of link width 28, link length 30 between contacts 24, and link pitch 32. This conventional laser link processing relies on heating, melting, and evaporating link 22, and creating a mechanical stress build-up to explosively open overlying passivation layer 44.
The thermal-stress explosion behavior is also somewhat dependent on the width of link 22. As the link width becomes narrower than about 1 xcexcm, the explosion pattern of passivation layers 44 becomes irregular and results in an inconsistent link processing quality that is unacceptable. Thus, the thermal-stress behavior limits the critical dimensions of links 22 and prevents greater circuit density.
U.S. Pat. No. 6,025,256 of Swenson et al. describes methods of using ultraviolet (UV) laser output to expose links that xe2x80x9copenxe2x80x9d the overlying passivation or resist material with low laser power through a different mechanism for material removal and provide the benefit of a smaller beam spot size. The links are subsequently etched.
U.S. Pat. No. 6,057,180 of Sun et al. describes methods of using UV laser output to remove links 22 positioned above a passivation layer of sufficient height to safeguard the underlying substrate from laser damage. This technique advocates modification of the target material and structure well in advance of laser processing.
Thus, improved link processing methods are still desirable.
An object of the present invention is, therefore, to provide a system or method that employs a single UV laser pulse to sever an IC fuse.
The present invention provides a Q-switched, diode-pumped, solid-state (DPSS) laser that employs harmonic generation through nonlinear crystals to generate green and/or IR and UV light. In a preferred embodiment, the type and geometry of the nonlinear crystals are selected to produce excellent beam quality suitable for subsequent beam shaping and focusing necessary to produce focused spot sizes that are advantageous for severing of IC fuses. The temperatures of the nonlinear crystals may also be precisely regulated using temperature feedback control loops to maintain advantageous phase matching conditions so as to produce uniform processing laser pulse characteristics. In addition, beam shape quality may also be enhanced by an imaged optics module capable of spatially filtering unwanted beam artifacts.
In a further preferred embodiment, because many standard alignment targets are difficult to detect with a UV laser beam, a fraction of the green or IR output may be utilized for the separate purpose of target alignment. The fractional green or IR target alignment beam follows a separate optical path with a separate set of optical elements and is attenuated to the proper power level. An imaged optics module for the fractional green or IR beam optimizes its shape for alignment scans. The green or IR alignment beam and the UV alignment beam pass through detection system modules and are separately aligned to a calibration target through a beam combiner common to both optical paths and their respective resulting reflected light is detected to calibrate the alignment beam with the UV link processing beam. The green or IR alignment beam can then be used to align the beam(s) to a given die, and the desired links on the die can be severed by the UV link processing beam without further calibration.
This invention provides the capability to produce high quality, focused spots that are smaller than conventionally used by IR link processing systems. The invention also provides improved UV pulse-to-pulse energy level stability while providing a means to deliver pulses at high repetition rates desired for improved throughput. This invention further provides a solution to the problem of aligning to alignment marks that have little contrast at the UV wavelength by using the green beam and/or IR beam, generated by the same source, as an alignment beam.
Additional objects and advantages of the invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.